Method of manufacturing an intrinsic polarizer

ABSTRACT

An improved method of forming an intrinsic polarizer, referred to as a K-type polarizer, includes stretching a polymeric film a first stretching step. The polymeric film comprises a hydroxylated linear polymer which is converted after the first stretching step to form dichroic, copolymer polyvinylene blocks aligned in the polymeric film. The polymeric film is stretched in a second stretching step while converting the hydroxylated linear polymer. This method produces an improved K-type polarizer with excellent polarizing and color characteristics. For example, the dichroic ratio is higher than 100, the color shift for light passed through the polarizer in a crossed configuration is low, and the absorption of light in the blue region of the visible spectrum is more than one half of the absorption for light in the middle of the visible spectrum.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Division of application Ser. No. 10/910,211 filed Aug. 3,2004. The disclosure of the prior application is hereby incorporated byreference herein in its entirety.

This application concerns the technical field of intrinsic polarizes,which is also addressed by the following U.S. patent applications:application Ser. No. 10/403,885, filed Mar. 31, 2003, entitled “PROCESSFOR MAKING AN INTRINSIC POLARIZER,” which is a continuation-in-part ofapplication Ser. No. 10/277,252 filed Oct. 20, 2002, entitled “ENHANCEDINTRINSIC POLARIZER”, which is a continuation-in-part of pendingapplication Ser. No. 10/118, 489, filed Apr. 6, 2002, entitled “ENHANCEDK-TYPE POLARIZER”, the relevant parts of which are incorporated byreference herein.

FIELD OF THE INVENTION

The invention relates to synthetic dichroic plane polarizers based onmolecularly oriented polyvinyl alcohol films and, in particular, to amethod of making a high efficiency intrinsic polarizing sheet or film.

BACKGROUND

Normally, light waves vibrate in a large number of planes about the axisof a light beam. If the waves vibrate in one plane only, the light issaid to be plane polarized. Several useful optical systems can beimplemented using polarized light. For example, in the manufacture ofelectro-optical devices such as liquid crystals display screens, crosspolarizers are used in conjunction with an addressable liquid crystalinterlayer to provide the basis for image formation. In the field ofphotography, polarizing filters have been used to reduce the glare andthe brightness of specular reflection. Polarizing filters, circularpolarizers or other optical components have also been used for glarereduction in display device screens.

Linear light polarizing films, in general, owe their properties ofselectively passing radiation vibrating along a given electromagneticradiation vector, and absorbing electromagnetic radiation vibratingalong a second given electromagnetic radiation vector, to theanisotropic character of the transmitting film medium. Dichroicpolarizers are absorptive, linear polarizers having a vectoralanisotropy in the absorption of incident light. The term “dichroism” isused herein as meaning the property of differential absorption andtransmission of the components of an incident beam of light depending onthe polarization direction of the incident light. Generally, a dichroicpolarizer will transmit radiant energy polarized along oneelectromagnetic vector and absorb energy polarized along a perpendicularelectromagnetic vector. A beam of incident light, on entering a dichroicpolarizer, encounters two different absorption coefficients, one low andone high, so that the emergent light vibrates substantially in thedirection of low absorption (high transmission).

SUMMARY OF THE INVENTION

Intrinsic polarizers are polarizers whose base material is converted toa dichroic material, and so a polarizing effect is produced without theneed to adsorb a dichroic material, such as iodine or dye, to the basematerial. Intrinsic polarizers, therefore, have a simpler construction,provide the possibility of being less expensive, are thinner and do notrequire the additional cover layers required by iodine or dyepolarizers.

One embodiment of the invention is directed to a method for making apolarizer from a polymeric film having an original length and comprisinga hydroxylated linear polymer. The method comprises stretching thepolymeric film a first stretching step and converting the hydroxylatedlinear polymer, after the first stretching step, to form dichroic,copolymer polyvinylene blocks aligned in the polymeric film. Thepolymeric film is stretched in a second stretching step while convertingthe hydroxylated linear polymer.

Another embodiment of the invention is directed to an intrinsicpolarizer that comprises a sheet of PVA-type matrix containing vinylenepolymer blocks. The sheet defines a pass polarization axis and a blockpolarization axis perpendicular to the pass polarization axis. Lighthaving an electrical vector parallel to the pass polarization axis issubstantially transmitted through the sheet and light having anelectrical vector parallel to the block polarization axis issubstantially absorbed in the sheet. The sheet exhibits a ratio, R,having a value of less than 2, where R is the ratio of the intrinsicabsorbance for light at 550 nm polarized parallel to the blockpolarization axis over the intrinsic absorbance for light at 400 nmpolarized parallel to the block polarization axis. The sheet alsoexhibits a polarization efficiency ratio in excess of 99%.

Another embodiment of the invention is directed to an intrinsicpolarizer that comprises a sheet of PVA-type matrix containing vinylenepolymer blocks. The sheet defines a pass polarization axis and a blockpolarization axis perpendicular to the pass polarization axis. Lighthaving an electrical vector parallel to the pass polarization axis issubstantially transmitted through the sheet and light having anelectrical vector parallel to the block polarization axis issubstantially absorbed by the vinylene polymer blocks. The sheet has anintrinsic absorption spectrum such that, when crossed with an identicalsheet and illuminated with a cold cathode fluorescent tube (CCFT) lightsource, the sheet transmits light having an a* co-ordinate with amagnitude of less than 2 and a b* co-ordinate with a magnitude of lessthan 2.

Another embodiment of the invention is directed to an intrinsicpolarizer that comprises a sheet of PVA-type matrix containing vinylenepolymer blocks. The sheet defines a pass polarization axis and a blockpolarization axis perpendicular to the pass polarization axis. Lighthaving an electrical vector parallel to the pass polarization axis issubstantially transmitted through the sheet and light having anelectrical vector parallel to the block polarization axis issubstantially absorbed by the vinylene polymer blocks. The sheetexhibits a dichroic ratio of more than 100.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates a method of manufacturing a polarizeraccording to principles of the present invention;

FIG. 2 presents a graph showing the absorption spectra for Samples 5-8discussed in Examples 2 and 3;

FIG. 3 presents a graph showing the standard spectrum for a cold cathodefluorescent tube (CCFT);

FIG. 4 presents a graph showing transmission of light as a function ofposition across the polarizer for a sample polarizer manufacturedaccording to principles of the present invention;

FIG. 5 presents a graph showing absorption of light in a polarizer as afunction of wavelength for polarizers converted under differentconditions;

FIG. 6 presents a graph showing transmission of light as a function ofwavelength through a single polarizer layer for different polarizersconverted under different conditions;

FIG. 7 presents a graph showing transmission through a pair of crossedpolarizers for different polarizers converted under differentconditions;

FIG. 8 presents a graph showing the transmission through the crossedpolarizers of FIG. 7, for light emitted from a CCFT light source havingan emission spectrum as shown in FIG. 3, and photopically corrected forthe response of the human eye;

FIG. 9 presents a graph showing the value of peak absorption and thevalue of the b* color co-ordinate for polarizers as a function ofconversion temperature

FIG. 10 presents a graph showing the absorbance in a single polarizersheet for four different types of polarizer sheets;

FIG. 11 presents a graph showing the absorbance in a pair of crossedpolarizer sheets for four different types of polarizer sheets; and

FIG. 12 presents a graph showing the crossed transmission through fourdifferent types of polarizers, for light emitted from a CCFT lightsource having an emission spectrum as shown in FIG. 3, and photopicallycorrected for the response of the human eye.

FIG. 13 schematically illustrates passage of light through an exemplaryintrinsic polarizer.

FIG. 14 schematically illustrates passage of light through a pair ofcrossed exemplary intrinsic polarizers.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to intrinsic polarizers, and is moreparticularly applicable to a method of making an intrinsic polarizerwith improved optically properties.

Examples of intrinsic polarizers include, for example, apolyvinylene-based polarizer such as a K-type polarizer. An intrinsicpolarizer derives its basic dichroism from the light-absorbingproperties of its matrix, rather than from the light-absorbingproperties of dye additives, stains, or suspended crystalline material,although additives such as dyes may be used to supplement the intrinsicdichroism. Typically, intrinsic polarizers comprise a sheet or film oforiented poly(vinyl alcohol)-type (PVA-type) material, having anoriented suspension of a dehydration product of PVA, polyvinylene in amatrix of PVA. Intrinsic polarizers of this kind are typically formed byunidirectionally stretching the polymeric film to align the PVA matrixand by heating the PVA-type polymeric film in the presence of adehydration catalyst, such as hydrochloric acid, to produce conjugatedpolyvinylene blocks. The formation of conjugated polyvinylene blocksfrom the polyvinyl alcohol is often referred to as “conversion.” Theoriented and dehydrated film may be referred to as “raw K”. By orientingthe PVA matrix unidirectionally, the transition moments of theconjugated polyvinylene blocks are also oriented, and the materialbecomes visibly dichroic. The conjugated polyvinylene blocks may bereferred to as dichroic chromophores. A boration treatment may beemployed after converting the polymeric film, as described in U.S. Pat.No. 5,666,223, the relevant parts of which are incorporated herein byreference.

The present invention relates to an enhanced intrinsic polarizer andmethod of making same in which improved polarizing properties areobtained. One embodiment of the method is directed to stretching the PVAfilm in a first stretching step, and then converting the film whilestretching the film in a second stretching step. The first stretchingstep may take place before, during, or after the film has been exposedto the dehydration catalyst.

The resulting polarizer comprises a composite of a molecularly orientedfilm of PVA/polyvinylene block copolymer material having thepolyvinylene blocks thereof formed by molecular dehydration of a film ofpolyvinylalcohol. The molecularly oriented film ofpolyvinylalcohollpolyvinylene block copolymer material comprises auniform distribution of light-polarizing molecules ofpolyvinylalcoholpolyvinylene block copolymer material varying in thenumber (n) of the conjugated repeating vinylene units of thepolyvinylene block of the copolymer. The value of n ranges from 2 toabout 25. The degree of orientation of the light polarizing moleculesincreases throughout the range with increasing values of n. The degreeof orientation of the molecules in conjunction with the concentrationdistribution of each polyvinylene block is sufficient to impart to thepolymeric sheet a photopic dichroic ratio (R_(D)), of at least 75.

Ignoring surface reflections, the photopic dichroic ratio, D, is definedas:

D=Az/A _(y)  (1)

Where A₂ and A_(y) are determined in the following manner. The samplepolarizer is illuminated with the sample beam of white light in a dualbeam spectrophotometer. The sample beam is pre-polarized using a highefficiency Glan-type polarizer. The amount of light transmitted throughthe sample polarizer at a particular wavelength is compared to theamount of light at the sample wavelength in the reference beam, and theabsolute absorbance of the sample polarizer is calculated as a functionof wavelength from the ratio of the transmitted light in the sample andreference beams. The absorbance is calculated over the range 380 nm-780nm. The absorbance spectra are sample polarizer and for light polarizedperpendicular absorbance spectra are then spectrally corrected for thespectrum of a particular light source and the response of the human eye(photopic correction). The integrated area under the corrected parallelabsorbance spectrum corresponds to the amount of spectrally correctedlight in the parallel polarization state that is absorbed in a singlepass through the sample polarizer, A_(y). The integrated area under thecorrected perpendicular absorbance spectrum corresponds to the amount ofspectrally corrected light, in the perpendicular polarization state,that is absorbed in a single pas through the sample polarizer, A_(z).

FIG. 13 schemtically illustrates passage of light through an exemplaryintrinsic polarizer disclosed herein. Intrinsic polarizer 1300 comprisespolyvinylalcohol-type matrix containing vinylene polymer 1301. Thepolymer blocks have been oriented by stretching. The polarizer is in theform of a sheet which defines x, y, and z axes, all of which areorthogonal. Cold cathode fluorescent tube 1302 emits light, depicted bysingle ray 1303, consisting of unpolarized light represented byelectrical vectors 1304 and 1305. Electrical vector 1304 is parallel tothe x axis also referred to as the block polarization axis. Electricalvector 1305 is parallel to the y axis also referred to as the passpolarization axis. The intrinsic polarizer transmits light, depicted bysingle ray 1306, which consists of polarized light represented byelectrical vector 1305 and 1307. Ideally, no light is absorbed along thepass polarization axis; electrical vector 1305 is shown unchanged afterpassing through the polarizer. It is generally desirable for all lightto be blocked along the block polarization axis, however, lighttypically leaks through the polarizer; thus, electrical vector 1304becomes reduced in magnitude to give electrical vector 1307 afterpassing through the polarizer.

FIG. 14 schematically illustrates passage of light through a pair ofcrossed exemplary intrinsic polarizers. An identical sheet of intrinsicpolarizer 1300 is placed adjacent the polarizer such that theorientations of the vinylene polymer blocks 1301 are generallyperpendicular to each other. Light depicted by single ray 1306 passesthrough the pair of crossed polarizers with transmitted lightrepresented by single ray 1308. Ideally, all light along the x and yaxes is blocked from being transmitted, however, light along these axestypically leaks through the pair of crossed polarizers; thus, electricalvectors 1304 and 1305 become reduced in magnitude to give electricalvectors 1310 and 1309, respectively. The transmitted light representedby single ray 1308 has an a* co-ordinate with a magnitude of less than 2and a b* co-ordinate with a magnitude of less than 2, wherein the a* andb* co-ordinates are measured according to the CIELAB color system.

One method for producing an enhanced intrinsic polarizer of the presentinvention is now described with respect to FIG. 1, which generally showsdifferent steps in the manufacturing process of a K-type polarizer. ThePVA-type film 100 is exposed to a dehydration catalyst 102, such as anaqueous acid solution, and is provided with a first stretch. The firststretch may take place before, during or after the film is exposed tothe dehydration catalyst. The film 104 is then converted to produce thedichroic chromophore and simultaneously stretched in a second stretchingstep 106. After conversion, the film may be borated 108, for example ina boration bath, and then dried and stretched in a third stretchingstep.

A support layer 110 may optionally be added to the film and/or strippedoff from the intrinsic polarizer film at various stages during themanufacturing process. In the illustrated embodiment, a support layer110 is optionally added either before or after the boration step 108.

Intrinsic Polarizer Starting Material

K-type polarizers use polymeric films derived from molecularly orientedpolyvinyl alcohol. Vinylalcohol polymers include any linear1,3-polyhydroxylated polymer or copolymer, or derivative thereof thatmay be dehydrated to a linear, conjugated vinylic polymer. Usefulvinylalcohol polymers include polymers and copolymers of units havingthe formula:

wherein R is H, a C₁-C₈ alkyl, or an aryl group; and R′ is H, or ahydrolysable functional group such as a C₁-C₈ acyl group. Preferably, Rand R′ are H. In addition to poly(vinyl alcohol) polymers andcopolymers, specifically contemplated are polyvinyl acetals and ketalsand esters as materials from which the molecularly oriented sheet orfilm can be formed. These are referred to a PVA-type materials. In thefollowing discussion, references to poly(vinyl alcohol) are understoodto cover polyvinyl acetals and ketals and esters, and references tovinylalcohol are understood to cover vinyl acetals, vinyl ketals andvinyl esters.

Useful co-monomers that may be polymerized with the vinylalcoholmonomers to produce vinylalcohol copolymers may include anyfree-radically polymerizable monomers including olefins, such asethylene, propylene and butylene, acrylates, acetylenes andmethacrylates such as methyl (meth)acrylate, vinyl acetates andstyrenes. Specifically contemplated for use in the present invention arecopolymers of ethylene and vinylalcohol. Generally, the amount ofco-monomer is less than 30 mole % and is preferably less than 10 mole %.Higher amounts may retard the formation of conjugated vinylene blocks(poly(acetylene) blocks) and deleteriously affect the performance of thepolarizer.

The preferred vinylalcohol polymers are homo- and copolymers ofpolyvinyl alcohol. Most preferred are polyvinyl alcohol homopolymers.Commercially available polyvinyl alcohols, such as those available fromCelanese Chemicals, Inc., Dallas, Tex., under the tradename CELVOL, areclassified by viscosity and percent hydrolysis. Polyvinyl alcoholshaving low viscosities are preferred for ease of coating, while having asufficiently high molecular weight to provide adequate moistureresistance and good mechanical properties.

Melt-processible polyvinyl alcohol may also be used in this invention.

The melt processible vinylalcohol polymers are plasticized to enhancetheir thermal stability and allow them to be extruded or melt-processed.The plasticizer can be added externally or may be part of thevinylalcohol polymer chain, in other words the plasticizer ispolymerized or grafted onto the vinylalcohol polymer backbone.

Vinylalcohol polymers that can be externally plasticized includecommercially available products such as “Mowiol” 26-88 and “Mowiol”23-88 vinylalcohol polymer resin available from Clariant Corp.,Charlotte, N.C. These “Mowiol” vinylalcohol polymer resins have a degreeof hydrolysis of 88%. “Mowiol” 26-88 vinylalcohol polymer resin has adegree polymerization of 2100 and a molecular weight of about 103,000.

Plasticizers useful in externally plasticizing vinylalcohol polymerinclude high boiling, water-soluble, organic compounds having hydroxylgroups. Examples of such compounds include glycerol, polyethyleneglycols such as triethylene glycol and diethylene glycol, trimethylolpropane, and combinations thereof. Water is also useful as aplasticizer. The amount of plasticizer to be added varies with themolecular weight of the vinylalcohol polymer. In general, theplasticizer will be added in amounts of between about 5% to about 30%,and preferably between about 7% to about 25%. Lower molecular weightvinylalcohol polymers typically require less plasticizer than highermolecular weight vinylalcohol polymers. Other additives for compoundingexternally plasticized vinylalcohol polymers include processing aids,i.e. Mowilith DS resin from Hoechst A. G., and anti-blocking agents,i.e., stearic acid, hydrophobic silica, colorants, and the like.

Externally plasticized vinylalcohol polymers are compounded by slowlyadding the organic plasticizer and typically water to the vinylalcoholpolymer powder or pellets under constant mixing until the plasticizer isincorporated into the vinylalcohol polymer, which occurs when the batchreaches a temperature of from about 82° C. (180° F.) to about 121° C.(250° F.). The lower the molecular weight of the vinylalcohol polymerresin, the lower the maximum batch temperature required to incorporatethe plasticizer. The batch is held at that temperature for about 5 to 6minutes. The batch is then cooled to about between 71° C. (160° F.) and93° C. (200° F.) at which time an antiblocking agent can be added. Thebatch is further cooled to about 66° C. (150° F.), at which time thevinylalcohol polymer granulates can be removed from the mixer andextruded.

The compounding steps used to externally plasticize the vinylalcoholpolymer can be eliminated when an internally plasticized vinylalcoholpolymer is made, except where it is desirable to add colorants, etc.Useful internally plasticized vinylalcohol polymers are commerciallyavailable. Such products include “Vinex” 2034 and “Vinex” 2025, bothavailable from Celanese Chemicals and Vinylon VF-XS available fromKuraray (Japan).

Materials available from Celanese under the Vinex trademark represents aunique family of thermoplastic, water-soluble, polyvinylalcohol resins.Specifically, the “Vinex” 2000 series including “Vinex” 2034 and “Vinex”2025 represent internally plasticized cold and hot water solublepolyvinylalcohol copolymer resins. Such internally plasticizedvinylalcohol copolymers are described in U.S. Pat. No. 4,948,857, hereinincorporated by reference. Such copolymers have the following generalformula:

where R is hydrogen or methyl;

R′ is a C₆-C₁₈ acyl group

y is 0 to 30 mole %;

z is 0.5 to 8 mole %; and

x is 70 to 99.5 mole %.

These copolymers retain the strength properties of poly(vinylalcohol)while also exhibiting increased flexibility. The acrylate monomerrepresented in the above formula gives the copolymer its internalplasticization effect. The degree of polymerization of the copolymerscan range from about 100 up to about 4000, preferably between about 2000and 4000. The degree of polymerization is defined as the ratio ofmolecular weight of the total polymer to the molecular weight of theunit as referenced in formula 2. Other internally plasticizedpoly(vinylalcohol) copolymer resins and preparation of these resins arediscussed in U.S. Pat. No. 4,772,663. “VINEX” 2034 resin has a meltindex typically of about 8.0 g/10 mins. and a glass transitiontemperature of about 30° C. (86° F.). “VINEX” 2025 resin has a meltindex typically of 24 g/10 mins. and a glass transition temperature ofabout 29° C. (84° F.).

Polyvinyl alcohols and copolymers thereof, are commercially availablewith varying degrees of hydrolysis, e.g., from about 50% to 99.5+%.Preferred polyvinyl alcohols have a degree of hydrolysis from about 80%to 99.5%. In general, a higher degree of hydrolysis, corresponds tobetter polarizer properties. Also, polyvinyl alcohols with a higherdegree of hydrolysis have better moisture resistance. Higher molecularweight polyvinyl alcohols also have better moisture resistance, butincreased viscosity. In the context of this invention, it is desirableto find a balance of properties in which the polyvinyl alcohol hassufficient moisture resistance, can be handled easily in a coating orcasting process and can be readily oriented. Most commercial grades ofpoly(vinylalcohol) contain several percent residual water andunhydrolyzed poly(vinyl acetate).

Coating of the dispersion/solution may be accomplished by a variety ofknown methods, including, for example, coating the substrate usingtechniques, such as shoe coating, extrusion coating, roll coating,curtain coating, knife coating, die coating, and the like, or any othercoating method capable of providing a uniform coating. The substrate maybe coated with a primer or treated with a corona discharge to helpanchor the polyvinyl alcohol film to the substrate. Suitable solutionbased primers are water-soluble copolyesters commonly used for primingpolyethylene terephthalate films such as those described in U.S. Pat.No. 4,659,523. After coating, the polyvinyl alcohol film is dried at atemperature typically from about 100° C. to 150° C. The thickness of thedried coating may vary depending on the optical characteristics desired,but is typically from about 25 μm to 125 μm (1-5 mils).

In another approach, the vinylalcohol polymer layer may bemelt-processed. As with solution coating, a melt comprising thevinylalcohol may be cast onto a substrate such as a carrier web orsupport layer. The vinylalcohol polymer film may also be melt-blown. Thevinylalcohol polymer melt may also be coextruded with the substrateusing a variety of equipment and a number of melt-processing techniques,typically extrusion techniques, well known in the art. For example,single- or multi-manifold dies, full moon feedblocks, or other types ofmelt processing equipment can be used, depending on the types ofmaterials extruded.

Stretching Steps

The manufacture of an enhanced intrinsic polarizing sheet or filmtypically begins with a polymeric film of a hydroxylated linear highPVA-type polymer having an original length, and generally having athickness on the order of 0.001 inches (25 μm) to 0.004 inches (100 μm).A suitable stretching device or other similar mechanism or system may beused to initially stretch the polymeric film from about 3.5 times toabout 7.0 times the original length of the polymeric film or greater.The first stretching step is typically conducted at a temperature abovethe glass transition temperature of the polymeric material.

The film may be stretched in a gaseous medium, such as air, or in aliquid medium, such as deionized water or an aqueous dehydrationcatalyst. When stretching in a gaseous medium, the film may be heated totemperatures, for example, in excess of 300° F. When the film isstretched before being dipped into a liquid medium, the stretching stepmay be referred to as a “dry stretch.”

When stretching in an aqueous medium, additional agents may be added toaid in the process, such as organic or inorganic salts, boric acidand/or borax, e.g., a surfactant, such as Triton X100 commerciallyavailable from Union Carbide, (Danbury, Conn.). Stretching in an aqueousmedium may also allow undesirable elements, such as glycerin, to leachout of the polymer film.

When the film is stretched in a liquid medium, the stretching step maybe referred to as a “wet stretch.” The film may also be stretched afterbeing removed from the liquid medium. The film typically has absorbedsome of the liquid and dehydration catalyst, if present, and so the stepof stretching after removing the film from the liquid medium may stillbe referred to as a “wet stretch.”

Stretching may be effected by the provision of heat generating elements,fast rollers, and slow rollers. For example, the difference in therotational rate between rollers may be exploited to create correspondingtension in the area of the sheet transported therebetween. When heatgenerating elements heat the sheet, stretching is facilitated and moredesirably effected. Temperature control may be achieved by controllingthe temperature of heated rolls or by controlling the addition ofradiant energy, e.g., by infrared lamps. A combination of temperaturecontrol methods may be utilized.

In unidirectional orientation, the film may be stretched without lateralrestraint from shrinking, or may be restrained from shrinking in thelateral direction. Such restraint may impose a small degree ofbidirectional orientation to the film. For example, a film may bestretched in a down-web direction and its lateral width maintainedconstant using a tentering apparatus.

Stretching may be performed at various stages throughout the filmmanufacturing process. Stretching that occurs before conversion isreferred to herein as a first stretching step, and may occur before thefilm is exposed to the dehydration catalyst, while the film is in thedehydration catalyst and/or after the film has been removed from thedehydration catalyst. Stretching that occurs simultaneously withconversion is referred to as a second stretching step and stretchingthat occurs after conversion, for example during or after a borationstep, is referred to as a third stretching step.

Support Layer

It may be desirable to cast, laminate or otherwise affix the polymericfilm onto a substrate such as a support film layer, heated roller, orcarrier web. A support layer, when bonded or otherwise affixed to thepolymer film provides mechanical strength and support to the article soit may be more easily handled and further processed. Some useful methodsof using a support layer are described in U.S. Pat. No. 5,973,834(Kadaba et al.), U.S. Pat. No. 5,666,223 (Bennett et al.) and U.S. Pat.No. 4,895,769 (Land et al.), the relevant portions of which areincorporated by reference.

If desired, the optional support layer may be oriented in a directionsubstantially transverse to the direction of orientation of thevinylalcohol polymer film. By substantially transverse, it is meant thatthe support layer may be oriented in a direction at least ±45° from thedirection of orientation of the vinylalcohol polymer film layer. Suchorientation of the support layer may provide greater strength in thetransverse direction than is provided by an unoriented support layer.

In practice, the support layer may be oriented before or after attachingto the vinylalcohol polymer layer. In one embodiment, the vinylalcoholpolymer may be oriented substantially uniaxially and bonded to anoriented support layer so that the directions of the orientations of thetwo layers are substantially transverse.

Any of a variety of materials can be used for the carrier web or supportlayer. Suitable materials include known polymeric sheet materials suchas the cellulose esters, e.g., nitrocellulose, cellulose acetate,cellulose acetate butyrate, polyesters, polycarbonates, vinyl polymerssuch as the acrylics, and other support materials that can be providedin a sheet-like form. Polyesters are especially useful, depending on theparticular application and the requirements thereof. A preferredpolyester is polyethylene terephthalate, available under the Mylar andEstar tradenames, although other polyethylene terephthalate materialscan be employed. In particular, one type of film that may be used as asupport layer is the Vikuiti™ brand DBEF type of reflective polarizerfilm, available from 3M Company, St. Paul, Minn.

The thickness of the support material varies with the particularapplication. In general, from the standpoint of manufacturingconsiderations, supports having a thickness of about 0.5 mil (0.013 mm)to about 20 mils (0.51 mm) can be conveniently employed.

Polarizing sheets or films made according to the present invention maybe laminated between or to supporting sheets or films, such as sheets ofglass or sheets of other organic plastic materials, and that lightpolarizers of the present invention either in laminated or unlaminatedform may be employed wherever other forms of light-polarizing plasticmaterials have been used, for example, in connection with sunglasses,sun visors, window pane glass, variable light transmission windows,glare masks, room partitions, and display devices such as liquid crystaldisplay panels, emissive display devices, cathode ray tubes, oradvertising displays.

Any of a variety of adhesives can be used for laminating the polarizingfilms onto other layers or substrates including polyvinyl alcoholadhesives and polyurethane adhesive materials. Inasmuch as the polarizerwill normally be employed in optical applications, an adhesive materialwhich does not have an unacceptable affect on the light transmissionproperties of the polarizer will generally be employed. The adhesivemay, on the other hand, include a colorant to produce a desired coloreffect. The thickness of the adhesive material varies with application.In general, thicknesses of about 0.20 mil (0.005 mm) to about 1.0 mil(0.025 mm) are satisfactory.

Exposing Film to Dehydration Catalyst

The PVA-type film is subjected to a conversion step, which may takeplace before or after bonding the vinylalcohol polymer to a supportlayer, or without any support layer. In the conversion step, a portionof the vinyl alcohol polymer in the polymeric film is converted topolarizing molecules of block copolymers of poly(vinylene-co-vinylalcohol). One approach to converting the vinyl alcohol is first toexpose the vinyl alcohol film to a dehydration catalyst and then to heatthe exposed film, thus causing dehydration to take place.

The film may be exposed to the dehydration catalyst in different ways.For example, the film may be dipped or immersed in an aqueousdehydration catalyst with sufficient residence time to allow thecatalyst to diffuse into the film. Other methods might include exposingthe film to acidic fumes containing the dehydration catalyst. Dippingthe polymeric film potentially allows higher processing speeds to beattained than with an acid fuming process since diffusion of aqueousspecies is faster in solution than in the gaseous state. In addition,the catalyst can be introduced to both sides of the polymeric film whenthe film is dipped in the catalyst. When exposing the film to acidicfumes, on the other hand, the film is typically exposed only on oneside. Accordingly, the dipping approach potentially provides a moreuniform concentration of the catalyst in the polymeric film, which mayimpact the cross-sectional distribution of dehydration chain lengths inthe resulting raw K film and provide a more balanced distribution ofchains.

The dehydration catalyst may be any acid or other agent which is capableof effecting in the presence of heat or other appropriate processingcondition the removal of hydrogen and oxygen atoms from the hydroxylatedmoieties of the linear polymer to leave conjugated vinylene units.Typical acids include hydrochloric acid, hydrobromic acid, hydroiodicacid, phosphoric acid, and sulphuric acid in methanol. The desireddegree of dehydration may vary, depending on the desired contrast andthe film thickness, but is typically in the range of 0.1 to 10%,preferably 1 to 5% of the available hydroxyl groups are converted tovinylene groups (i.e., —CH₂—CHOH—→—CH═CH).

For example, the polymeric film may be immersed in an aqueoushydrochloric acid solution for about one second to several minutes. Inanother example, the polymeric film may be immersed in deionized waterfor about one second to about five minutes and then immersed in anaqueous hydrochloric acid solution for about one second to severalminutes. The concentration of the aqueous hydrochloric acid solution ispreferably about 0.01 Normal to about 4.0 Normal.

The dehydration step may also be achieved by other methods, such as bycoating the oriented sheet with an acid coating and then subjecting itto a heating step to effect the dehydration of the polymeric sheet, orby coating the oriented sheet with an acid donor layer. In the latterexample, a photoacid generator or a thermal acid generator is dissolvedor dispersed in the donor layer and, upon irradiation with a radiantenergy, the incipient acid diffuses into the adjacent vinylalcoholpolymer matrix to effect partial dehydration of the vinylalcohol polymerto conjugated vinylene [poly(acetylene)] segments. The radiant energymay be thermal energy or ultraviolet light energy, depending on the typeof acid generator used.

Processing agents may be added to the acid to aid in the process, suchas organic or inorganic salts, boric acid and/or borax, e.g., asurfactant, such as Triton X100 commercially available from UnionCarbide, (Danbury, Conn.).

Conversion

After exposing the film to the dehydration catalyst, PVA-type film andthe adsorbed catalyst may then be heated, whereby the oriented film isconverted into the desired dehydration product, polyvinylene. The filmmay be heated through conduction heating, convection heating, radiationheating, or a combination thereof. The conversion process results in theconverted film giving up water and the acid catalyst in the form ofvapor.

For example, the polymeric film and the catalyst may be passed through aheating oven with a temperature range of from about 88° C. to about 205°C. for about a few seconds to about ten minutes. In another approach,the film and catalyst may be exposed to microwave radiation heating orto laser heating.

Another method of converting the film is to expose film and catalyst toradiant infrared heating, for example generated using an infraredheating lamp or lamps, from about one second to about sixty seconds.Infrared heating potentially allows higher processing speeds to beattained than with hot air impingement methods. In addition, infraredheating allows for a rapid startup and shutdown of the conversionprocess. Furthermore, when heating is effected using a number of radiantheaters placed across the film, it may be possible to achieve lanewisecontrol of the conversion process by individually controlling the amountof radiation emitted from the different radiant heaters.

Variations in the temperature and duration of the dehydration heatingstep may affect the optical properties of the finished polarizer.Considerable latitude in process parameters exists without detriment tothe formation of the copolymer and its concomitant polarizationproperties. There is a balance among time, temperature and acidconcentration for a given optical property. For example, the extent ofpenetration of the acid into the film may be controlled by altering thetemperature of the acid solution, altering the residence time of thefilm in the acid, and/or altering the concentration of the acid. Forexample, a lower transmission polarizer may be achieved at a giventemperature by using longer immersion times. At a given immersion time,lower transmission may by achieved at higher temperatures. Generally, itis preferred that the diffusion of dehydration catalyst within the filmreaches equilibrium. If a high transmission polarizer is desired, loweracid concentrations are preferred. If a lower transmission polarizer isdesired then higher acid concentrations may be used.

The film may be subjected to a second stretching step during theconversion process. In other words, the film may be stretched a secondtime while the conversion process is occurring. This second stretchingstep may result in an increase in the film length by up to about 2.5times the intermediate length of the film obtained after the firststretching step. Like the first stretching step, the second stretchingstep occurs at a temperature above the glass transition temperature ofthe polymeric material, and may be effected by the provision of heatgenerating elements, fast rollers, and slow rollers.

Boration

The polymeric film may also be subjected to a boration step followingconversion, in which the oriented film is borated, for example byexposing the converted film to an aqueous boration solution. Theboration step effects relaxation and cross-linking. A third stretchingstep may be carried out before, during, or after the polymeric film isborated. For example, the polymeric film may be submerged and allowed tosoften and/or swell in an aqueous boration solution. This often resultsin relaxation, or shrinkage, of the film. The film is subsequentlyremoved and dried. The film may receive a third stretch during and/orafter drying following the boration step. In another approach, thepolymeric film may be stretched when still submerged into the borationsolution.

The boration step may employ one or more baths. For example, in atwo-bath boration treatment, the first bath may contain water, and thesecond, a boric ion contributing species. The order of the baths may bereversed or both baths may contain varying concentrations and/ormixtures of boric ion contributing species. Stretching and/or relaxationof the polymeric film may be conducted in any one or more of thesebaths.

The boration solution generally comprises boric acid. In addition, theboration solution may comprise either sodium or potassium hydroxide, ormay include a substance from the class consisting of the sodium andpotassium borates, preferably borax. The concentration of boric acid andborax or other borate in the solution or solutions to which the orientedpolarizing film is subjected may vary. Preferably, the boric acid ispresent in a higher concentration than the borax or other borate, andthe solutions may contain from about 5% to about 20% by weight of boricacid and from 0% to about 7% by weight of borax. A preferredconcentration ranges from about 6%-16% by weight of boric acid and from0%-3% by weight of borax.

The polarizing sheets or films may be immersed in a boration solution orsolutions for a period of about one minute to about thirty minutes andpreferably maintained at about 50° C. or higher. A preferred borationtemperature ranges from about 70° C. to about 110° C. Boration of themolecularly oriented polymeric film is subject to considerablevariation. For example, the temperature of the boration solution may bevaried, and the concentration thereof may be increased at highertemperatures. It is desirable that the solution be heated to at least50° C. or greater in order to accomplish rapid “swelling” andcross-linking of the sheet.

Following exposure to the boron-containing solution, the polarizingsheet may be rinsed and dried. The sheet may be rinsed using anysuitable method, such as passing the sheet through a bath of de-ionizedwater, or by spraying de-ionized water on the sheet. The sheet may bedried by heating the sheet, for example through convection or radiationheating. In one approach, the sheet may be passed through a convectionoven.

Processing agents may be added to the boration bath to aid in theprocess, for example, a surfactant such as Triton X100 commerciallyavailable from Union Carbide, (Danbury, Conn.).

The polarizing sheet typically shrinks during the boration step, if notleft under tension. Allowing the polarizing sheet to shrink permits thepolarizer sheet to take up more boron-containing solution, and thusleads to a higher degree of cross-linking, with a concomitant increasedenvironmental stability. The polarizing sheet may be restretched afterboration. For example, the sheet may be stretched in a third stretchingstep up to about 120% of the shrunk length. The restretching may beperformed while the sheet is still in the boration bath or after it hasbeen removed from the boration bath. For example, if the boration stepis followed by rinsing and drying, the restretch may take place in adeionized rinsing bath or while being dried.

Subsequent to the second stretching step and/or boration step, theresulting intrinsic polarizer may be bonded or laminated to an optionalsupport layer. The optional layer may be the same or different from anoptional support layer previously stripped off.

The process of wet-stretching, conversion and boration can be applied tothe PVA-type film as a continuous, integrated process. Such a continuousprocess is simpler than the multistep processes that have been used forintrinsic polarizers in the past, and leads to higher film yield andreduced polarizer cost.

To further illustrate the present invention, the following Examples areprovided, but the present invention is not to be construed as beinglimited thereto. Unless otherwise indicated, all parts, percents andratios are by weight. In the Examples, unpolarized light transmissionwas measured on the raw K samples by passing a beam of white lightthrough the sample, through a photopic filter, and then through aphoto-detector. Unpolarized light transmission on raw K samples for anintrinsic polarizer typically ranges from 15% to about 50%. Thepolarizing efficiency was calculated according to the following equationby determining the transmittance with axes parallel (T_(par)) which wasdetermined by overlapping the sample polarizer with the high efficiencypolarization analyzer in such a manner as to make the axes thereofparallel with each other, and the transmittance with axes crossed(T_(perp)), which was determined by overlapping the same in such amanner as to make the axes at right angles to each other:

Polarizing efficiency,η(%)=(T _(par) −T _(perp))/(T _(par) +T_(perp))×100  (3)

Unless otherwise indicated, all Examples used an aqueous borationsolution having a 9%-12% boric acid concentration and a 3% boraxconcentration.

EXAMPLE 1 Birefringent Characteristics

Four samples of PVA-type film were stretched by different amounts, aslisted in Table I. These samples were prepared for measuring thebirefringence that results from the orientation of the PVA moleculeswhen the PVA film is stretched. As can be seen, the amount ofbirefringence, which is related to the degree of orientation, increaseswith increased stretching. Also, the film become thinner with increasedstretching.

TABLE I Sample Stretch (%) Birefringence Thickness (μm) 1 400 0.033731.4 2 650 0.0400 23.9 3 750 0.0418 21.1 4 850 0.0438 17.8

Each film was 2400 DP PVA, available from Kuraray Co. Ltd., Osaka,Japan., contained about 12% glycerin plasticizer and, before stretching,had a thickness of 75 μm and a width of 26″ (66 cm). All samples werestretched by 400% in a first, wet stretch step, and were thensubsequently stretched by different amounts in a second stretch stepunder IR illumination. For Samples 1-4, the wet stretch took place indeionized water, with no acid present. Accordingly, there was nodehydration catalyst, and so no conversion took place when the film wasexposed to the IR lamp. In all cases, the IR lamp was a ProthermInfrared Heater, FS Series, medium wavelength, available from ProcessThermal Dynamics, Inc., Brandon, Minn., unless indicated otherwise. Theconditions under which Samples 1-4 were made are listed in Table II.

TABLE II Sample Manufacturing Conditions Sample No. 1 2 3 4 Input linespeed (cm/s) 0.5 0.25 0.25 0.25 Water bath temperature (° C.) 36.7 36.736.7 36.7 Wet Stretch (1^(st) step, %) 400 400 400 400 Water bath pathlength (cm) 86 86 86 86 IR lamp temperature (° C.) 445 550 563 568 IRfilament, distance from film (cm) 20 20 20 20 IR lamp length (cm) 25 2525 25 Reflector distance from film (cm) 7.5 7.5 7.5 7.5 IR stretch(2^(nd) step, %) 100 163 187 213 Total stretch (%) 400 650 750 850

Apart from Sample 1, all samples had an input line speed of 0.25 cm/sec,corresponding to 0.5 feet/minute. Sample 1 was transported twice asfast, with a line speed of 0.5 cm/sec (1 foot/minute). The film waspassed into a water bath held at a temperature of 36.7° C. The pathlength in the water was 86 cm (34 in). The samples were each stretchedby 400% in the water. A value of stretch indicates the ratio of thelength after stretching to the length before stretching. Thus, a stretchof 400% indicates that a length of 1 unit of film was stretched to alength of 4 units.

The films were then removed from the water tank and exposed to IR lightfrom an IR lamp whose temperature could be adjusted. The IR lamp had areflector behind the heating element to reflect heat to the film. In allcases the distance between the reflector and the film was 7.5 cm. Thedifferent samples were stretched by different amounts when exposed tothe IR lamp. Sample 1 was not stretched when exposed to the IR lamp, asindicated by the stretch amount of 100%. The films were unsupportedduring the wet stretch and IR stretch steps.

EXAMPLE 2 Polarization Characteristics

Four more samples were prepared for measuring how the polarizationperformance of the film is dependent on the amount of stretching.

TABLE III Pol. eff. Sample Stretch (%) Kv (%) (%) D 5 400 42.4 98.1351.6 6 650 42.3 99.88 87.5 7 750 42.5 99.96 107.4 8 850 42.5 99.94 101.7

Samples 5-8 were prepared like Samples 1-4 in Example 1 above, exceptthat the water bath contained acid. Therefore, for Samples 5-8, theexposure to the IR lamp resulted in conversion of the film. Thetransmission for upolarized light (Kv) was calculated from transmissionmeasurements through the films made using a spectrophotometer (CaryModel No. 5E). Kv is the average transmission through the film of lightpolarized parallel to the transmission axis (T_(par)) and of lightpolarized perpendicular to the transmission axis (T_(perp)). Thepolarizing efficiency was calculated using expression (3) providedabove.

The dichroic ratio, D, was defined in expression (1) above. The resultslisted in Table III show that the polarization coefficient and thedichroic ratio generally increase for increased amounts of stretching,while maintaining a substantially constant value transmission for lightin the pass polarization state. The values of polarization coefficientand dichroic ratio for Sample 8, however, are a little less than thosefor Sample 7. The conversion conditions and boration step conditionsaffect the optical characteristics of the resultant polarizer film. Theconversion and boration conditions for Sample 8 had not been optimized.It is believed that the polarization performance of a film stretched by850% may be increased with optimization of boration conditions.

The polarization efficiencies of Samples 7 and 8 are both high, inexcess of 99.94%, and the dichroic ratios are both higher than 100. Thusthe polarization properties of Samples 7 and 8 are significantlyimproved over previously obtained values for KE polarizers.

The conditions under which Samples 5-8 were made are listed in Table IV.

TABLE IV Sample Manufacturing Conditions Sample No. 5 6 7 8 Input linespeed (cm/s) 0.5 0.25 0.25 0.25 Water bath temperature (° C.) 31 37.8 3535 Wet Stretch (1^(st) step, %) 400 400 400 400 Water bath path length(cm) 86 250 86 86 IR lamp temperature (° C.) 445 550 560 570 IRfilament, distance from film 20 20 20 20 (cm) IR lamp length (cm) 25 2525 25 Reflector distance from film (cm) 7.5 7.5 7.5 7.5 IR stretch(2^(nd) step, %) 100 163 187 213 Boration tank temperature (° C.) 8587.8 90.5 93.3 Boric acid conc. (%) 11.25 11.9 11.2 11.2 Sodium boratedecahydrate 3.0 3.0 3.0 3.0 (borax) conc. (%) Boration relax (%) 0 5 1013 Restretch (3^(rd) stretch) during rinse 108 106 106 106 and dry (%)Total stretch (%) 430 655 720 790

After the second stretch step, during conversion, the films were placedinto a boration bath containing boric acid and sodium borate decahydrate(Na₂B₄O₇.10H₂O, also known as borax). The films were permitted to relaxin the boration bath, resulting in a shrinkage of as much as 13% inlength. The films were rinsed in water and dried in a convection ovenafter being removed from the boration bath. The films were restretched(3^(rd) stretch step) during the rinse and dry process. The films wereunsupported by a support layer throughout the fabrication process.

EXAMPLE 3 Color Characteristics

The color characteristics of Samples 5-8 were calculated for lighttransmitted through a single layer and also for a light transmittedthrough a pair of crossed layers (transmission axes perpendicular),using the wavelength dependent absorbance measurements made using theusing the spectrophotometer. The absorbance spectra for the differentsamples are shown in FIG. 2. The absorbance spectra for Samples 5-8 arerespectively labeled as curves 205, 206, 207 and 208 in FIG. 2. The hueof the transmitted light was subsequently calculated, and is listed inTable V. Unless otherwise stated, the hue is calculated for illuminationusing a cold cathode fluorescent tube (CCFT), as is commonly used in LCDdisplays for e.g. laptop computers. The spectrum of the CCFT source ispresented in FIG. 3, normalized so that the total intensity integratedover the wavelength range shown, the area under the curve, is equal toone. Color, or hue, is presented according to the CIELAB color system,which uses three co-ordinates: L*, a* and b*. The L* co-ordinate isrelated to lightness, the a* co-ordinate represents red/green color andthe b* co-ordinate represents yellow-blue color. A positive value of a*corresponds to red and a negative value of a* corresponds to green. Apositive value of b* corresponds to yellow and a negative value of b*corresponds to blue. The (a*,b*) co-ordinate of (0,0) represents aneutral hue, black, grey or white, depending on the value of the L*co-ordinate. Furthermore, a value of a* or b* whose magnitude is lessthan 1 results in a barely perceptible change in color from neutral. Thecolor characteristics are presented for a single sheet, for a pair ofsheets oriented with the transmission axes parallel and for a pair ofsheets with the transmission axes crossed.

TABLE V Color Characteristics Single Single Parallel Parallel CrossedCrossed Sample a* b* a* b* a* b* 5 −0.891 8.593 −2.410 15.081 16.9788.834 6 −0.160 4.720 −0.261 9.045 2.475 −2.252 7 −0.501 3.475 −0.8536.544 0.556 −0.461 8 −0.113 3.219 −0.113 6.140 1.025 −1.074

Sample 5 has relatively poor color characteristics, since the values fora* in the crossed configuration and for b* in both configurations are solarge. Samples 6-8, on the other hand have significantly improved colorcharacteristics. Both Samples 7 and 8 show values of a* and b* whosemagnitudes are less than 2 and, in fact, are less than 1, for Sample 7in the crossed configuration. Sample 7, in particular, shows a neutralhue in the crossed configuration and a slightly blue hue in the singlelayer configuration. In addition, Sample 8 shows a substantially neutralhue in the crossed configuration and a more neutral hue in the singlelayer configuration than Sample 7. In the parallel configuration, bothSamples 7 and 8 show a hue of less than one a* unit, and show reasonablemagnitudes of b* of less than 7. These color characteristics arerelatively neutral because the concentration of conjugated vinyleneblocks is relatively constant over a large range of n, where n is thenumber of vinyl units conjugated in the polyvinylene block, as isdiscussed in U.S. patent application Ser. No. 10/277,252, incorporatedherein by reference.

Samples 7 and 8, therefore, provide excellent polarizationcharacteristics, as listed in Table III, while also providing excellentcolor characteristics. Such color neutrality has not previously beenobtainable in intrinsic K-type polarizers of high polarization qualitywithout the aid of extrinsic chromophores such as dyes.

EXAMPLE 4 Uniformity of Polarization and Color Characteristics

The uniformity of the optical characteristics across the film(cross-web) was measured for a film made under the conditions listedabove for Sample 7, i.e. a total stretch of 720%. The stretched film hada width of about 9 inches (23 cm). The transmission, Kv, polarizingco-efficiency, dichroic ratio and color were measured at steps of oneinch (2.5 cm) from one of the edges of the film. The results are shownin Tables VI and VII.

TABLE VI Polarization Characteristics vs. Film Position Distance fromPolarizing co- edge (cm) Kv (%) efficiency (%) D 2.5 42.3 99.95 97.4 542.9 99.96 92.5 7.5 41.9 99.96 90.8 10 42.3 99.96 98.6 12.5 42.2 99.9697.3 15 42.4 99.96 103.4 17.5 42.4 99.95 101.6 20 42.7 99.92 103.2

The values for Kv are also shown in the graph presented in FIG. 4. Kvvaries by about ±0.6% across the width of the film. The polarizingefficiency varies by about ±0.02% across the film and the dichroic ratiovaries by about ±5 across the film. The variation in the polarizationcharacteristics across the film is relatively small.

TABLE VII Color Characteristics vs. Film Position Distance from SingleSingle Crossed Crossed edge (cm) a* b* a* b* 2.5 −0.188 2.603 0.918−1.235 5 −0.252 3.233 0.438 −0.426 7.5 −0.465 3.499 0.425 −0.352 10−0.433 3.417 0.594 −0.460 12.5 −0.404 3.655 0.537 −0.395 15 −0.518 3.8540.555 −0.279 17.5 −0.466 4.073 0.565 −0.234 20 −0.4101 3.506 0.952−0.680

The color characteristics vary by only small amounts over the film. Forexample, other than one value close to the edge, all the hues in thecrossed configuration have a magnitude of less than one, which isminimally perceptible to the human eye, if perceptible at all. The onlyvalue greater than one, (b* at 2.5 cm from the edge) is greater than oneby only a small amount, and is barely perceptible to the human eye. Inthe single layer configuration, the magnitudes of a* all remain lessthan one, while the magnitudes of b* vary between 2.603 and 4.073.

EXAMPLE 5 IR Lamp Temperature

The effect of the lamp temperature on polarization and colorcharacteristics was investigated. Samples 9-12 were fabricated under thesame conditions as listed above for Sample 7, except that the lamptemperature was varied and the boration conditions were: boric acidconcentration ˜9.5% and sodium borate decahydrate concentration ˜3%.

The absorbance (optical density) and transmittance of the differentpolarizer sheets were calculated from measurements made using the dualbeam spectrophotometer. The resulting absorbance and transmittance forsingle polarizer sheets are shown in FIGS. 5 and 6 respectively. Theabsorbance is the intrinsic absorbance, and results from intrinsicabsorption by the polyvinylene blocks in the film itself, and not fromabsorption of species added to the film, such as iodine or dye. Theabsorbance curves for samples 9-12 are labeled in FIG. 5 as curves 509.510, 511 and 512 respectively. Also, the transmission curves in FIG. 6for samples 9-12 are labeled respectively as curves 609, 610, 611 and612.

The absorbance for Samples 10 and 11 is particularly high in the blueregion of the spectrum, a region of the spectrum that has previouslyseen relatively low absorbance for K-type polarizers using previousmethods of construction. In particular, the ratio, R, of the absorbanceat 550 nm over the absorbance at 400 nm is around 1.54, showing that theabsorbance at the blue end of the spectrum is around two-thirds of theabsorbance in the middle of the spectrum. Thus, Samples 10 and 11 bothshow that the ratio, R, is less than 2, and is less than 1.7.

The transmission through a pair of crossed polarizer sheets is shown inFIG. 7 for the different polarizer samples. The transmission curves forsamples 9-12 are labeled respectively as curves 709, 710, 711 and 712.The transmission plotted on the y-axes in FIGS. 6 and 7 is the absolutevalue of transmittance. Thus, a transmission value of 0.1 indicates that10% of the incident light is transmitted through the polarizers. Whenthe data of FIG. 7 are convoluted with a CCFT light source, whosespectrum is given in FIG. 3, and photopically corrected for the responseof the human eye, the transmission spectrum of the crossed polarizers isas shown in FIG. 8. The photopically corrected transmission curves forsamples 9-12 are respectively labeled as curves 809, 810, 811 and 812 inFIG. 8.

The polarization characteristics for Samples 9-12 are shown in TableVIII, the transmission characteristics are given in Table IX and thecolor characteristics are given in Table X.

TABLE VIII Polarization Characteristics Lamp Polarizing Contrast Sampletemp. (° C.) co-eff. (%) D Ratio 9 550 99.79 98.6 463 10 560 99.89 115.7914 11 570 99.89 116.0 873 12 580 98.94 76.6 93

The contrast ratio is defined as the ratio of the parallel transmissionover the crossed transmission, listed below in Table IX. It should benoted that, concomitant with the relatively low value of R, thepolarizing efficiency exhibited by Samples 10 and 11 is greater than99.8%, while the dichroic ratio, D, for both samples was greater than110.

TABLE IX Transmission Characteristics (%) Single Sample (Kv) ParallelCrossed 9 43.0 36.9 0.0796 10 43.2 37.3 0.0408 11 43.2 37.3 0.0427 1243.2 37.0 0.3946

The transmission characteristics are integrated over the visiblespectrum for the polarizers illuminated with the CCFT standard lightsource and photopically corrected for the response of the human eye. Thevalues of crossed transmission correspond to the areas under the curvesshown in FIG. 8.

TABLE X Color Characteristics single single par. par. crossed crossedSample a* b* a* b* a* b* 9 0.117 1.311 −0.210 3.253 5.509 −8.814 10−0.651 3.793 −1.206 7.200 1.602 −0.780 11 −0.720 4.022 −1.317 7.5891.590 −0.374 12 −0.034 3.210 −0.771 6.127 10.207 −0.037

The color characteristics listed in Table X correspond to the calculatedhues produced under illumination by light from the standard CCFT lightsource that has passed through the particular polarizer configuration.Thus, “single” refers to the hue of the light that is transmittedthrough a single layer of the polarizer sample, “par.” refers to the hueof the light that has passed through a stack of two layers of thepolarizer sample with the transmission axes parallel, and “crossed”refers to the hue of the light that has passed through a stack of twolayers of the polarizer sample whose transmission axes areperpendicular.

As can be seen from inspection of Tables VIII-X, and FIGS. 5-8, theperformance of the polarizer peaks when the lamp temperature used inthis particular manufacturing process is in the region of 560° C.-570°C. Of particular note is the performance of the polarizer in the blueportion of the spectrum. Previously, high values of A_(z) and low valuesof T_(z) in the blue portion of the spectrum were not achieved withintrinsic polarizers. The processes of wet stretching and simultaneousstretching and conversion result in improved blue performance, with lowtransmittance and high absorbance. The absorbance is greater than 2through the blue region (400 nm-500 nm), and is greater than 3 for somewavelengths in the range 400 nm-450 nm. Accordingly, the change is colorwhen used in a crossed configuration is relatively small. In particular,Samples 10 and 11 both exhibit values of a* and b* whose magnitudes areless than 2, and the value of b* is less than 1.

Lamp temperatures outside of the range 560° C.-570° C. resulted inincreased blue transmission, for example as is shown in FIG. 6 for thecurves corresponding to a lamp temperature of 550° C. and 580° C. Thisincreased transmission may indicate that the number of shorter vinyleneblocks (low n) being conjugated during the conversion process is lessthan at temperatures around 560° C.-570° C. Furthermore, at higher thanoptimal temperatures, e.g. 580° C., there is increased transmission oflight in the red region of the spectrum. This is shown in the departureof the curves corresponding to 580° C. from the other curves in theregion of 600 nm-700 nm. This may indicate that the number of longervinylene blocks (high n) being conjugated is reduced when thetemperature is higher than optimum.

EXAMPLE 6 Control of “Blue Leak”

Traditional methods of preparing K-type polarizers suffer from aso-called “blue-leak”, in the crossed state absorption spectrum, whereabsorbance drops to relatively low values for wavelengths below about450 nm. The currently accepted way of increasing the blue absorption isto add a blue-absorbing dye to the intrinsic polarizing film. The datapresented under Example 5, however, suggest that the blue absorption canbe controlled to some extent by the temperature and power of the IR lampused in the conversion process. Thus, use of the manufacturing processdiscussed above provides the ability to control the yellow-blue, orb*-axis, color of the resulting polarizer film. This is due to themodulation of the dehydration chain length distribution of thechromophore in the film, with a larger relative ratio of short chainlengths (low n) providing the increase in blue absorption.

Since there is no requirement to add a blue absorbing dye, it is easierto control the manufacture of the resulting polarizer, as it is only theheating source that is controlled, and there is no need to preciselycontrol the adsorption of the blue-absorbing dye. Additionally, theresulting polarizer provides improved environmental stability since theabsorbing chromophores are intrinsic to the PVA matrix, and there is nodye adsorbed on the polarizer film surface.

To explore the blue performance of the K-type polarizer further, anumber of samples were prepared using the following method. A cast filmof polyvinyl alcohol, 75 μm thick, containing ˜12% wt. glycerinplasticizer and with approximately 2400 average degree ofpolymerization, was stretched by a factor of 650% while being passedthrough a ˜0.05 Normal aqueous solution of hydrochloric acid at atemperature of 52° C. After stretching, the film was passed through anIR heating zone where the film was heated, thus causing conversion ofsome of the PVA to polyvinylene blocks. While the film was continuouslyprocessed in this manner, the power applied to the IR heating zone wasincreased, and the temperature of the heater was monitored using athermocouple. The geometry of the heater was different from that used inExamples 1-5, so the temperatures of the IR heater discussed in theprevious examples do not necessarily correspond to those of thisexample.

As the heating temperature was increased, the resulting, un-boratedpolarizer was sampled. The un-borated polarizer was measured spectrallyin the two orthogonal absorption axes (y and z) by orientation with acalcite crystal analyzer in a UV-visible spectrophotometer. Theresulting A_(z) and A_(y) spectral curves were used to calculate theparallel and crossed state color of the raw polarizer samples andwavelength of maximum absorption (λ_(max)) of the A_(z) component as afunction of IR temperature.

The results are shown in the graph in FIG. 9. The value of λ_(max)(solid curve) reduces as the temperature increases, indicating that theconcentration of smaller conjugated vinylene molecules (low n) increaseswith increasing temperature. Also shown in FIG. 9 is a curve (dashedline) that shows the value of b* for crossed polarizers as a function ofIR temperature. The value of b* is close to zero for this particularmanufacturing process at a temperature of about 645° C.

EXAMPLE 7 Comparison with Different Types of Polarizers

The performance of a polarizer manufactured according to the processesdiscussed here, and referred to as Wet KE, or wet-stretch KE polarizer,was compared to that of other types of polarizers. The wet-stretch KEpolarizer was manufactured under conditions similar to those for Sample10.

The other types of polarizers included an iodine polarizer, a dye stuffpolarizer and a dry stretch KE polarizer (Dry KE). The iodine polarizerhad a layer of PVA with adsorbed iodine molecules, sandwiched betweentwo layers of cellulose triacetate (TAC), and was taken from a Sharpmodel 13B2UA LCD television, supplied by Sharp Electronics Corp, Mahwah,N.J. The dye stuff polarizer had a layer of PVA with adsorbed dichroicdyes, sandwiched between two layers of TAC, and was taken from a Philipsactive matrix display, model no. LTE072T, supplied by Philips ConsumerElectronics North America, Atlanta, Ga.

The dry stretch polarizer was a K-type polarizer made using a processthat included a 7× dry stretching step at 182° C. After dry stretchingstep, the film was exposed to hydrochloric acid vapors and dehydrated byheating the fumed film in an oven at a temperature in excess of 125° C.,for example as discussed in U.S. Pat. No. 5,666,223.

The film was dipped into a first boration bath, held at a temperature of80° C., that contained a solution of 7% boric acid and 3% borax. Thefilm relaxed in length by 10% when in the first bath. The film was thendipped in a second boration bath, at a temperature of 88° C., thatcontained a solution of 9.5% boric acid and 3% borax. The film wasstretched to a ratio of 1.15 in the second boration bath and thenstretched by another ratio of 1.06 after removal from the secondboration bath, for an overall stretch ratio of 7.7. The film was driedfollowing the final stretch.

The absorbance of each polarizer, as a function of wavelength, wascalculated from measurements made using the Cary Model 5Espectrophotometer. The absorbance (optical density) for each polarizeris shown in FIG. 10, as a function of wavelength, for light polarizedparallel to the transmission axis of each polarizer. The absorbancecurves for the iodine, dyestuff, dry stretch and wet stretch polarizersare respectively numbered as curves 1002, 1004, 1006 and 1008 in FIG.10. The absorbance (optical density) spectrum for each polarizer isshown in FIG. 11 for light polarized perpendicular to the transmissionaxis of each polarizer. The absorbance curves for the iodine, dyestuff,dry stretch and wet stretch polarizers are respectively numbered ascurves 1102, 1104, 1106 and 1108 in FIG. 11. Polarization andtransmission characteristics for the four different types of polarizersare listed in Table XI, and the color characteristics are listed inTable XIII.

TABLE XI Polarization and Transmission Characteristics Polarizing Cont.Single Parallel Crossed Polarizer co-eff. (%) D Ratio (Kv) (%) (%) (%)Iodine 99.98 120.0 4411 42.7 36.5 0.0083 Dye 99.85 37.4 686 37.8 28.60.0416 Dry KE 99.86 90.9 731 42.5 36.1 0.0494 Wet KE 99.96 110.9 243442.7 36.4 0.0149

TABLE XII Color Characteristics single single par. par. crossed crossedPolarizer a* b* a* b* a* b* Iodine −2.054 4.356 −3.699 8.119 0.165−0.480 Dye −0.948 3.484 −1.618 6.289 −0.063 −0.079 Dry KE 0.442 0.8810.278 2.784 6.840 −12.767 Wet KE −0.457 3.245 −0.806 6.160 0.778 −0.816

The dye stuff polarizer absorbs significantly more light in the passpolarization state than for the other three types of polarizer, shown bythe high absorbance in FIG. 10. This gives rise to the significantlylower values of Kv and parallel transmission for the dye stuff polarizerlisted in Table XI. Both the iodine and dye stuff polarizer have layersof TAC, which absorbs light in the blue: this explains the substantiallyidentical absorbance in FIG. 10 for the iodine and dye stuff polarizersfor wavelengths less than about 430 nm. Both the wet-stretched anddry-stretched KE polarizers, on the other hand, absorb less light in thewavelength range below about 450 nm than the iodine and dye stuffpolarizers. It is believed that this relatively low value of blue lightabsorption is due to the absence of TAC layers in the KE polarizer.

Considering now the absorption curves presented in FIG. 11, which showabsorption of light polarized perpendicular to the transmission axis,the iodine polarizer shows the highest value of absorption (OD about4.5) for wavelengths in the range 400 nm-700 nm. The wet-stretch KEpolarizer, on the other hand, has a maximum OD of about 4, anddemonstrates levels of absorption similar to that for the iodinepolarizer for wavelengths between about 400 nm and 550 nm. Theabsorption of the wet-stretch KE polarizer is significantly higher inthe range 400 nm-550 nm than for the dry-stretch KE polarizer. Thisconfirms that the process used to manufacture the wet-stretch KEpolarizer results in increased numbers of short chain (low n)polyvinylene blocks.

Review of the polarization and transmission characteristics listed inTable XI shows that the wet-stretch polarizer displays betterperformance than the dyestuff polarizer and the dry-stretch polarizer,and is comparable in most characteristics to the iodine polarizer. Also,in terms of the color characteristics, listed in Table XII, the wetstretch KE polarizer shows performance that is as color neutral as, ifnot more neutral than, that of the iodine polarizer. In particular, thehue of light transmitted through parallel polarizers is more neutralwith the wet-stretch KE polarizer than the iodine polarizer, for both a*and b*. More particularly, the magnitude of a* is less than one for thewet-stretch KE polarizer, compared with a magnitude of more than 3 forthe iodine polarizer. Also, the value of b* (6.1599) is less for thewet-stretch KE polarizer than for the iodine polarizer (8.1189).

For the crossed polarizer configuration, the magnitudes of a* and b* areboth slightly less for the iodine polarizer than for the wet-stretch KEpolarizer. However, the magnitudes of a* and b* for the wet-stretchpolarizer are both less than one, which means that there is noperceptible hue, or only a barely perceptible hue, for the wet-stretchpolarizer. The color characteristics of the dye stuff polarizer aresimilar to those of the wet-stretch polarizer for parallel and crossedconfigurations, but the transmission and polarization characteristics ofthe dye stuff polarizer are not as good as those of the wet-stretch KEpolarizer.

The absorption of light polarized perpendicular to the transmission axisis higher in the red end of the spectrum for both the iodine and dyestuff polarizers than for either the wet-stretch KE polarizer or thedry-stretch polarizer, as is seen in FIG. 11. This is not a significantadvantage, however, for applications where the light passing through thepolarizer is to be viewed by the human eye and/or where the light sourceilluminating the polarizer has low output in the red portion of thespectrum.

When the wavelength dependent response of the human eye and commonlyused light sources are considered, the performance of the wet-stretch KEpolarizer is similar to that of the iodine polarizer. FIG. 12 shows thephotopically corrected transmission for crossed polarizers for the fourdifferent types of polarizers, where the illumination source is assumedto be the standard CCFT source. The photopically corrected transmissioncurves for the iodine, dyestuff, wet stretch and dry stretch polarizersare respectively labeled 1202, 1204, 1206 and 1208. The area under eachcurve corresponds to the photopically corrected transmission through thecrossed polarizer pair. Both the dye stuff and dry-stretch polarizersallow significant leakage of light at the 550 nm peak, and thedry-stretch polarizer also transmits significant amounts of light in theblue region of the spectrum. The transmission through crossedwet-stretch polarizers is almost exactly the same as through crossediodine polarizers, except for small differences at about 580 nm and 610nm. These differences, however, are small, as is evidenced by the factthat magnitudes of a* and b* are both less than one for the crossed,wet-stretch polarizer.

Thus, a KE polarizer manufactured in accordance with the descriptionherein shows transmission, polarization and color properties that aresubstantially the same as iodine polarizers. KE polarizers are intrinsicand, unlike iodine or dye stuff polarizers, do not require theadsorption of dichroically absorbing species, and do not require coverlayers, such as TAC, for environmental stability. Thus, a practical KEpolarizer may be made thinner than either iodine or dye stuffpolarizers, has a less complex structure than iodine or dye stuffpolarizer, and is less expensive to manufacture than iodine or dye stuffpolarizers. Furthermore, the wet-stretch polarizers are more able towithstand conditions of high humidity than iodine or dye stuffpolarizers.

Intrinsic polarizers as described herein may be used with other layers.For example, a polarizer may be used with a substrate to providestructural support, or may be used with a liquid crystal display. Theother layers used with the polarizer may be isotropic or may bebirefringent.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

1. A method for making a polarizer from a polymeric film having anoriginal length and comprising a hydroxylated linear polymer, the methodcomprising: stretching the polymeric film a first stretching step;converting the hydroxylated linear polymer, after the first stretchingstep, to form dichiroic, copolymer polyvinylene blocks aligned in thepolymeric film; and stretching the polymeric film in a second stretchingstep while converting the hydroxylated linear polymer.
 2. A method asrecited in claim 1, wherein the hydroxylated linear polymer is at leastone of polyvinyl alcohol, polyvinyl acctal, polyvinul ketal andpolyvinyl ester.
 3. A method as recited in claim 1, further comprisingexposing the polymeric film to a dehydration catalyst before stretchingthe polymeric film in the second stretching step.
 4. A method as recitedin claim 3, wherein exposing the polymeric film to the dehydrationcatalyst immersing the polymeric film in a bath containing thedehydration catalyst.
 5. A method as recited in claim 3, wherein thedehydration catalyst is an aqueous catalyst comprising at least one ofhydrochloric acid, hydrobromic acid, hydroiodic acid, phosphoric acid,and sulphuric acid in methanol.
 6. A method as recited in claim 3,wherein dehydration catalyst comprises hydrochloric acid with aconcentration ranging from about 0.01 Normal to about 4.0 Normal.
 7. Amethod as recited in claim 3, further comprising stretching thepolymeric film in the first stretching step while exposing the polymericfilm to the dehydration catalyst.
 8. A method as recited in claim 7,further comprising immersing the polymeric film in the dehydrationcatalyst and stretching the polymeric film in the first stretching stepwhile immersed in the dehydration catalyst.
 9. A method as recited inclaim 3, further comprising stretching the polymeric film in the firststretching step after exposing the polymeric film to the dehydrationcatalyst.
 10. A method as recited in claim 3, further comprisingstretching the polymeric film in the first stretching step beforeexposing the polymeric film to the dehydration catalyst.
 11. A method asrecited in claim 3, wherein converting the hydroxylated linear polymercomprises heating the polymeric film to effect partial dehydration ofthe polymeric film, thereby forming vinylene block segments.
 12. Amethod as recited in claim 11, further comprising converting thepolymeric film by heating the polymeric film after exposing thepolymeric film to the dehydration catalyst.
 13. A method as recited inclaim 12, wherein heating the polymeric film comprises exposing thepolymeric film to infrared radiation.
 14. A method as recited in claim12, wherein heating the polymeric film comprises convectively heatingthe polymeric film.
 15. A method as recited in claim 1, wherein thesecond stretching step is bidirectional unrelaxed, unidirectionalunrelaxed or parabolic.
 16. A method as recited in claim 1, wherein thefirst stretching step is bidirectional unrelaxed, unidirectionalunrelaxed or parabolic.
 17. A method as recited in claim 1, wherein thefirst stretching step comprises stretching the polymeric film to anintermediate length in the range from about 3.5 to about 7 times theoriginal length.
 18. A method as recited in claim 17, wherein the secondstretching step comprises stretching the polymeric film to a stretchedlength of about 1.1 to 2.5 times the length of the intermediate length.19. A method as recited in claim 1, wherein stretching in the first andsecond stretching steps combines to stretching the polymeric film fromabout 4 times to about 8.5 times the original length.
 20. A method asrecited in claim 1, further comprising borating the polymeric film aftersecond stretching step.
 21. A method as recited in claim 20, whereinborating the polymeric film comprises exposing the polymeric film to aborating solution containing at least boric acid.
 22. A method asrecited in claim 21, wherein the solution contains from about 5% toabout 20% by weight boric acid.
 23. A method as recited in claim 21,wherein the solution also contains one of a sodium borate and apotassium borate.
 24. A method as recited in claim 23, wherein thesolution contains from about 0% to about 7% by weight sodium boratedecahydrate.
 25. A method as recited in claim 21, wherein the boratingsolution has a temperature in excess of 50° C.
 26. A method as recitedin claim 25, wherein the borating solution has a temperature in therange from about 70° C. to about 110° C.
 27. A method as recited inclaim 21, further comprising drying the polymeric film after exposingthe polymeric film to the borating solution containing at least boricacid.
 28. A method as recited in claim 20, further comprising permittingthe polymeric film to shrink to a borated length while borating thepolymeric film.
 29. A method as recited in claim 28, further comprisingstretching the polymeric film in a third stretching step afterpermitting the polymeric film to shrink while borating the polymericfilm.
 30. A method as recited in claim 29, wherein stretching thepolymeric film in the third stretching step comprises stretching thepolymeric film up to about 120% of the borated length.